1. Field of the Invention
The invention relates to a liquid crystalline fiber.
2. Description of the Prior Art
There has been considerable effort to develop human-made actuator materials that can mimic muscle performance. The developmental goal is to generate large mechanical actuation induced by external stimuli such as electric field, temperature, and light. Many materials and approaches have been developed towards this goal including hydrogels, dielectric elastomers, shape memory polymers, conducting polymers, carbon nanotubes, and ferroelectric liquid crystal elastomers. However, there are few that come close to meeting the properties of natural muscle. For instance, hydrogels show a very large volume change. The drawback is a low modulus and speed. On the other hand, electrostrictive materials have demonstrated a very fast response rate, but a high voltage is required.
Because of their anisotropic orientational symmetry in combination with rubber elasticity, liquid crystal (LC) elastomers are promising materials for applications in the field of sensors and actuators. The potential for liquid crystalline materials to exhibit unusual properties was first suggested by de Gennes, Phys. Lett., 28A, 725 (1969). (All referenced publications and patents are incorporated herein by reference.) Subsequently, such elastomers have been prepared and their resultant properties investigated. In general, the elastomers most frequently studied have been those based on side-chain liquid crystalline polymers rather than the main-chain systems considered originally by de Gennes. These elastomers exhibit anisotropic shape change under applied fields as they go through phase transitions and retain network memory, which enables them to reversibly contract and extend.
There are two basic approaches to prepare LC elastomers: the first approach developed by Mitchell and co-workers (Lacey et al., J. Mater. Chem., 8, 53 (1998)) involves crosslinking an acrylate polymer prealigned in a magnetic field. Such samples are found to show complete recovery from their global orientation on cooling to the nematic phase from the isotropic phase. The second method due to Finkelmann and co-workers (Kundler et al., Macromol. Chem. Phys., 199, 677 (1998)) involves a two-step cross-linking strategy of a siloxane liquid crystal polymer. The first stage involves a lightly cross-linking of the polymer while applying a stress field. Subsequently, a second cross-linking reaction is performed which fixes the uniaxial alignment. By this method LC elastomers of large dimensions with permanent alignment and highly anisotropic mechanical properties were produced. An alternative approach to the use of chemical reactions to produce intermolecular cross-linking is photo-crosslinking. Although such materials show promise for the generation of elastomers, there may be a number of problems associated with their use.
Thomson et al., (Macromolecules, 34, 5868 (2001)) presented detailed studies of mechanical properties of two LC elastomer films. These networked films exhibited muscle-like physical properties with strains of 35-40% and blocked stress values of the order of 200 kN/m2.